The invention relates to a signal cable for transmission of optical signals in oil and gas wells.
Signal cables are i.a. used in oil and gas wells for measurement of such parameters as pressure, temperature, tension, flow, and for fibre optic communication. The cables may have a length of 6 km or more for use in wells, and should tolerate temperatures of 200xc2x0 C. and pressure over 1000 bar. In addition, the cables are vulnerable to corrosion because they are usually located in the liquid filled annulus between the casing and the tubing.
NO patent-publication 175119 describes a cable for this application, where the fibre is inside a steel tube which protects it from the hydrostatic pressure which normally will be several hundred bar. The tube is stranded around a centre element, and the fibre is able to move sidewise in the tube so that some elongation of the cable due to thermal expansion and/or mechanical tension, is possible without straining the fibre.
For submarine cables, it is known that as a result of corrosion of, or at, the metal-containing cable, hydrogen is produced. The hydrogen diffuses into the optical fibre and results in attenuation of the light being transmitted.
The attenuation can be permanent or reversible. Permanent losses are a result of a reaction between the hydrogen which diffuses into the optical fibre, and the silica glass, and the loss will generally increase with the hydrogen concentration, and with time. Reversible losses are the results of hydrogen molecules which diffuse into the optical fibre, but does not combine with the glass. The latter loss is proportional with the concentration of hydrogen.
Representative partial pressures of hydrogen in submarine cables are about 1 bar. For submarine cables, one has tried to solve this problem by, for example, placing the optical fibre cable inside a copper tube in order to establish a barrier against diffusion of hydrogen into the optical fibre.
In oil and gas wells it has surprisingly been found that considerable amounts of hydrogen are developed in the area outside and on the cable. Because of the high temperature prevailing in such wells, normally 100-200xc2x0 C., the hydrogen will quickly penetrate into a steel tube which encapsulates the fibre, and diffuse into the optical fibre and cause losses. The partial pressure of hydrogen inside the pipe can be up to 100 bar. Even at much lower hydrogen pressures, the loss in the fibre will normally be so high that the cable is no longer usable for the intended purpose.
The use of copper in such applications is, however, not practical, because of the high hydrostatic pressure to which the cable is exposed. Copper will further not be a particularly effective barrier against hydrogen at the high temperatures of interest.
One object of the invention is to provide a signal cable for transmission of optical signals in oil and gas wells, said cable preventing or reducing the penetration of hydrogen into the fibre material.
Another object of the invention is to provide such a device which also reduces the formation of hydrogen in the area around the fibre optical cable in oil/ gas wells.
The invention relates to a signal cable for transmission of optical signals in oil and gas wells, said cable comprising one or more optical fibres encapsulated in at the least one pressure and temperature resistant hollow and normally tube- or channel-shaped protection means, optionally placed by or integrated with a cable-construction, for example as described in NO patent publication 175119.
According to the invention, the protection means is provided by a thin continuous surface coating of gold, or a gold-alloy which mainly comprises gold.
According to the invention the surprising technical effect resulting from this new gold coating on fibre optic signal cables is firstly that the gold coating passivates the metal in the cover means resulting in a strong reduction in the formation of hydrogen on the surface of the cover means. Secondly, the solubility of hydrogen in gold is very low, and therefore the permeability rate for hydrogen in gold is very low. With a goldcoating on the cover means, for example a steel tube, with a few micrometers thickness, for example 1-10 xcexcm, it is therefore possible to achieve a satisfactory and lasting signal transmission in the fibre optical cable for a period for several years, even with ambient hydrostatic pressure of several hundred bar, ambient temperature around 200xc2x0 C., and in corrosive environments.
Gold coating with such thickness can be established at the cover means with known methods, such as electroplating, on tube-lengths of several kilometers.
The gold coating is preferably provided at the exterior surface of the cover means, both in order to passivate the cover material, and to establish a diffusion barrier against hydrogen. The gold coating may however, as an alternative or in addition, also be provided at the interior surface of the cover means, to establish a diffusion barrier against hydrogen.
The signal cable according to the invention, may further comprise several cover means which encapsulate each other, for example concentric pipes or channels of steel, and one or more of the said surfaces of the concentric placed cover means, may be provided with gold coating. It is preferably the exterior surface of the cover means which is provided with a gold coating. This produces both a passivating effect and a barrier. A gold coating on the interior surface will only produce a barrier effect and besides be harder to apply than an exterior coating.
The gold coated signal cable according to the invention, may be used in different cable constructions, for example, the one described in NO patent-publication 175119. In order to protect the gold coating on the signal cable according to the invention, during production, installation and operation, it would be advantageous to provide a thin protecting layer on the outside of the cover means, for example of a temperature resistant plastic, before the gold coated signal cable enters into the production process for the composite cable. Examples of applicable plastics are polyether keton (PEEK), perfluor alkoxy (PFA) and teflon.
A plastic coating as described above, provided on the exterior surface of the cover means or at the exterior surface of the outermost cover means, may also have a passivating effect on the surface of the cover means and reduce local production of hydrogen.
The thickness of the gold coating can be selected considering the range of use and signal cable construction. Multiple gold coatings demand less thickness per coating, and lower pressure/ temperature in the surroundings also demands less thickness. Normally, the thickness of the coating should not be less than 2-3 micrometers in order to achieve a coating without pores. In the example below it is given an example of a calculation with a 3 micrometers thickness of the coating.